Hypothetical resonance structure is invalid due to orthogonality of orbitals:
$$CH_2=CH-Cl\not\leftrightarrow CH_2^+ - CH=Cl^-$$The Cl lone pair in an spΒ² orbital cannot overlap with the C=C Ο-system, so the βM effect is negligible and the C(spΒ²)βCl bond remains strong.